Internal combustion engines have contributed greatly to the advancement of society. Vehicles powered by these engines have shortened the travel times between us by making long distance road travel routine. Such engines, however, have also greatly contributed to the pollution of our environment. The combustion of petroleum products in these engines results in unwanted byproducts such as carbon monoxide, carbon dioxide, sulfur dioxide, nitrogen dioxide, etc., that are dumped into our atmosphere.
Vehicles powered by alternative energy sources are under development. One such alternative energy source is the fuel cell. Fuel cells, for example, generate electrical power through electrochemical reaction of a fuel and oxidant, such as for example, hydrogen and oxygen. The electrical power that is generated is used to drive an electric traction motor that, in turn, drives the wheels of the vehicle. The product of the electrochemical reaction in a fuel cell utilizing hydrogen and oxygen is water, a product which is easily disposed of.
FIG. 1 illustrates one embodiment of a hydrogen and oxygen supply for a fuel cell stack that, for example, is used to power an electric vehicle. The oxygen for the fuel cell reaction can typically be obtained from the ambient air while the hydrogen is obtained, for example, from a hydrogen fuel tank, a hydrogen storage device, or in a reformate stream from a catalytic reformer. The hydrogen and air for the fuel cell stack are handled by respective air and hydrogen supply systems that are each under the control of, for example, a programmable logic controller (PLC).
The power available from the fuel cell stack must be adjusted to meet, as far as possible, the power required to run the various electrical loads of component systems of the vehicle (for example, in addition to the traction motor, loads may include air conditioning equipment, lights, pump motors, etc.). The fuel cell reaction, and, thus, the available output power from the fuel cell stack, may be controlled by regulating the air mass flow from the air supply system through the stack.
A fuel cell based electric power generation system with an improved reactant supply and control system is described in U.S. Pat. No. 5,366,821, which is incorporated herein by reference in its entirety. In particular, U.S. Pat. No. 5,366,821 describes a system in which the reactant pressure, mass flow, utilization ratio and the temperature may be regulated, independently or interrelatedly, to increase net fuel cell power output under fluctuating load conditions. In a preferred embodiment of the system, a receiver is used to dampen pressure fluctuations, and to store and provide additional reactant to the fuel cell as needed, during periods of fluctuating power demand.
In a fuel cell powered vehicle, the air supply system needs to respond rapidly to highly and rapidly varying power demands, but because of space constraints it is undesirable to employ an air receiver in the system.
In the system illustrated in FIG. 1, the air flow is under the control of a compressor within the air supply system. The compressor, in turn, is under the control of the PLC and appertaining circuitry used to control the speed of the compressor based on various sensed input signals. These sensed input signals include the measured air mass flow of the air supplied to the fuel cell, as measured by sensor 10 of FIG. 1, the current output from the fuel cell stack, as measured by current sensor 15, the voltage output of the fuel cell stack, as measured across output leads 20, and accelerator pedal movement and position, as measured by system 25.
One manner in which the PLC can use the foregoing input signals to control the illustrated system is set forth in FIG. 2. As would be understood by those skilled in the art, the PLC performs the illustrated steps and functions under a combination of hardware and software control.
The PLC accepts the sensed fuel cell current value and calculates the air mass flow that is needed to provide the power required by the loads from the fuel cell stack based on the sensed current value. Additionally, the PLC determines whether changes to the air mass flow are needed as a result of changes in the accelerator pedal position and to what degree such changes are required. Still further, the PLC determines whether an additional increase in air mass flow is required as a result of a low voltage condition of the fuel cell stack based on the measured fuel cell stack voltage. The results of these three calculations are summed and are compared to the measured air mass flow to generate an error signal. This error signal is processed, for example, using a PID (proportional-integral-derivative) control, such a control being understood and readily implemented by those skilled in the design of control systems. PID processing results in an output correction signal value that, for example, may be converted to an analog signal by an digital-to-analog converter, that is supplied to control a mechanism, for example, the speed controller of the air compressor, to provide the corrected air mass flow to the fuel cell stack.
In the foregoing system, any increase in air compressor speed also results in an increase in the sensed fuel cell current, because of the additional current drawn by the compressor. This results in a positive feedback loop to the PLC's fuel cell current input signal. The positive feedback, in turn, causes a change in the output correction signal that is supplied to the air compressor and causes the speed of the air compressor to change. This in turn causes the sensed fuel cell current to change again, thus rendering the system unstable and causing unnecessary revving of the air compressor.
Another system that describes the control of air flow through a fuel cell stack in response to fuel cell current is set forth in U.S. Pat. No. 5,434,016 issued Jul. 18, 1995, entitled "Process And Apparatus For Supplying Air To A Fuel Cell System", which is incorporated by reference herein in its entirety. This system likewise may be rendered unstable by the influence of the compressor current on the sensed fuel cell current input to the controller.
The present control system and method provides the high dynamic response required in a fuel cell powered vehicle, where the fuel cell power output must react quickly to rapidly changing power demands. Feed-forward control is used to smooth oxidant supply and reduce system instability. Feedback control may also be used to make fine adjustments to the oxidant supply. Adaptive control techniques may be used to adjust subsequent feed-forward control signals in response to varying operating system conditions. Further, in situations where the power demands of the electrical loads exceed the desired maximum fuel cell power output threshold, power management techniques may be used to control and limit the power distribution to the various electrical loads.